The transformers package

Haskell 98 part of a monad transformer library, inspired by the paper
"Functional Programming with Overloading and Higher-Order Polymorphism",
by Mark P Jones, in Advanced School of Functional Programming, 1995
(http://web.cecs.pdx.edu/~mpj/pubs/springschool.html).

This part contains the monad transformer class, the concrete monad
transformers, operations and liftings. It can be used on its own
in Haskell 98 code, or with the monad classes in the monads-fd or
monads-tf packages, which automatically lift operations introduced
by monad transformers through other transformers.